It has been recognized that it is desirable to provide a slant plate type of compressor, such as a wobble plate type piston compressor, with a displacement or capacity adjusting mechanism to control a compression ratio in response to demand. In the wobble plate type piston compressor, control of the compression ratio can be accomplished by changing a slant or incline angle of a sloping surface of a slant plate to a drive shaft in response to crank chamber pressure which is controlled by a pressure control mechanism such as disclosed in U.S. Pat. No. 4,428,718 issued Jan. 31, 1984 to Timothy J. Skinner. In this wobble plate type piston compressor, the slant plate stops in any incline angle when the compressor is stopped, and also starts wobble motion in any angle when the compressor is started. The compressor can be seriously damaged when operated in this manner, particularly when the compressor is used in an automotive air conditioning system. For example, if rotation of the slant and wobble plates is initiated at a high speed by an engine of a vehicle through an electromagnetic clutch with the slant plate situated at the largest slant angle with respect to the longitudinal axis of the drive shaft, the complex components of the compressor, such as the variable displacement mechanism, a rotation-preventing mechanism of the wobble plate and seal elements which are disposed in a cylinder head receive a sudden and large shock. Furthermore, this shock is increased by operation of the compression of suction refrigerant gas including a large amount liquified refrigerant gas. As a result, these interior components of the compressor can be seriously damaged.
U.S. Pat. No. 4,543,043 issued Sept. 24, 1985 to Richard W. Roberts discloses the two types of devices to avoid the disadvantages of allowing the slant plate to stop in any position. One device is shown in FIG. 6 and another device is shown in FIG. 2 of the '043 U.S. patent.
The device illustrated in FIG. 6 uses a piston-stroke-decreasing bias spring mounted on a drive shaft. The spring is located between a rear surface of a thrust flange, i.e. the rotor, and a front surface of a hinge ball. The piston-stroke-decreasing bias spring provides a force tending to move a wobble plate-drive plate assembly, i.e., slant plate, mounted on the hinge ball toward a minimum piston stroke position. Such a prior art mechanism exhibits the following problems: the compressor always starts at a minimum piston stroke stage, because the piston-stroke-decreasing bias spring urges the wobble plate-drive plate assembly, including a stop pin, to the minimum slant angle. When the compressor is started at a minimum piston stroke stage, only minimal compression gas force is generated tending to increase the slant angle. In addition, an excessive compression gas force in the cylinder is needed to oppose the restoring force of the piston-stroke-decreasing bias spring. Therefore, it takes a relatively long time to obtain a proper slant angle in relation to the heat load of the compressor.
The device illustrated in FIG. 2 of the '043 patent includes both a piston-stroke-decreasing bias spring and a piston-stroke-increasing bias spring. The piston-stroke-decreasing bias spring is mounted on the drive shaft at a location between the rear surface of the thrust flange, i.e. the rotor, and the front surface of the hinge ball. The piston-stroke-increasing bias spring is mounted on the drive shaft at a location between a rear surface of the hinge ball and a cylinder block. The bias forces of two springs tend to move the hinge ball along the drive shaft in opposite directions. However, at an equilibrium balanced position, the hinge ball is positioned to provide a nominal stroke of about 0.100 inch to pistons. The two spring system overcomes the problems relating to above single spring device, by the use of the piston-stroke-increasing bias spring. However, other problems arise. For example, a complicated structure requiring a bias spring on both sides of the slant plate must be assembled. This complicated structure makes the step of compressor assembly more difficult and costly. Another problem, which occurs during displacement changes, is an unusual vibration of the slant plate at a natural frequency of the bias springs' applying forces in opposite directions on the slant plate.
Roberts '043 discloses a capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive axis, nutates but does not rotate, and drivingly couples the pistons to the drive source. This type of capacity adjusting mechanism, using selective fluid communication between the crank chamber and the suction chamber, however, can be used in any type of compressor which uses a slanted plate or slanted surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604, issued to Terauchi, discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used herein to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or slanted surface in the drive mechanism.